1. Field of the Invention
The present invention relates to a semiconductor microwave amplifier based on a microstrip line mainly using a surface-mounted semiconductor amplification device for which automatic mounting or reflow is easy to perform.
2. Description of Prior Art
Conventionally, since a semiconductor microwave amplifier of this type uses a small semiconductor amplification device, the input and output electrodes of the semiconductor amplification device are close to each other, so high-frequency isolation can hardly be obtained. Consequently, the deviation in frequency characteristics of the gain increases, and additionally, little output power is obtained, or the output power is positively fed back to the input to easily cause oscillation.
Techniques for solving these problems have been developed. In a microwave semiconductor device disclosed in Japanese Examined Patent Publication No. 63-63121 (Japanese Unexamined Patent Publication No. 57-141944), the semiconductor amplification device is covered with a metal member having an opening with a predetermined shape, thereby obtaining desired isolation.
In a microwave amplifier disclosed in Japanese Unexamined Utility Model Publication No. 1-122618, a surface-mounted field effect transistor is used as the amplification device to form a microstrip high-frequency amplification circuit. In this microwave amplifier, the source electrode is grounded using a through hole. However, since the through hole has an inductance, the return loss degrades, the gain decreases, and oscillation or a decrease in output power due to positive feedback occurs. To prevent these disadvantages, the inductance of the through hole must be lowered. For this purpose, a large-sized through hole is used, and it is formed as close as possible to the source electrode to decrease the excess distribution constant between the source electrode and the ground surface. This measure also provides an effect for connection between the ground electrode and the ground surface in use of a semiconductor amplification device other than the field effect transistor.
This measure is effective for the above-described microwave semiconductor device disclosed in Japanese Examined Patent Publication No. 63-63121 as far as the semiconductor amplification device allows direct mounting on the ground surface. However, in use of a semiconductor amplification device to be surface-mounted on a printed-circuit board, when the semiconductor amplification device is to be covered with a metal member, the structure becomes complex because the printed circuit board must be subjected to special working such as hole formation for metal member attachment or connection for obtaining the ground surface. Additionally, in forming a microstrip amplification circuit, the metal member must be connected to the ground electrode at a low impedance. However, this process is difficult to perform for the surface-mounted semiconductor amplification device because of its shape.
In the microwave semiconductor device disclosed in Japanese Examined Patent Publication No. 63-63121, the holes formed in the printed-circuit board degrades isolation. Therefore, if high-frequency isolation is required between the upper and lower surfaces of the printed-circuit board, and an internal layer ground surface is formed, the ground surface for the microstrip line is formed in the multilayered printed-circuit board, so the above measure cannot be applied.
Furthermore, in the microwave semiconductor device disclosed in Japanese Examined Patent Publication No. 63-63121, a general automatic mounting machine for surface-mounted components performs only two-dimensional component mounting and does not cope with such three-dimensional assembly. For this reason, surface-mounted components can hardly be automatically mounted.
In the microwave amplifier disclosed in Japanese Unexamined Utility Model Publication No. 1-1226181, a large through hole must be formed near the ground electrode to decrease the impedance. However, if device soldering is performed using reflow, the solder flows into the through hole. A solder necessary for fixing the ground electrode may also flow into the through hole, resulting in degradation in soldering of the ground electrode. For this reason, the connection impedance between the ground electrode of the semiconductor amplification device and the ground surface for the microstrip line can hardly be lowered because of the structure.